Since the off-the-road tires used for construction vehicle and the like are required to have considerably high load capacity and traction performance as compared with tires for truck and bus, it is general to have a tread pattern that ribs capable of developing a high wear resistance under a high load are formed on a central portion of the tread and lugs capable of developing a high traction performance are formed on both side portions of the tread.
However, when a width of a zone forming straight or zigzag ribs in the central portion of the tread sandwiching between phantom lines each connecting tops of lug grooves to each other is made wider than necessary, traction performance during the running on an uphill grade or the like is lacking, and also there is a drawback that lateral slippage is apt to be easily caused in the steering because circumferential groove(s) is not naturally existent in the central portion. For this end, so-called lug-block mix pattern in which block rows producing an edge effect in a widthwise direction of the tread are formed in the central portion of the tread are used at the present time.
On the other hand, the tire is recently and strongly demanded to have further improvements of the load capacity and traction performance and an improvement of wear resistance with the further advance of vehicle performances. In order to satisfy such a demand, there is a tendency that the tire size is made large and the thickness of the tread is made thicker. In this case, it is particularly feared that heat generation in the central portion of the tread exerts upon the durability of the tire.
For the purpose of obtaining a tread pattern capable of being durable to excessive traction and realizing an excellent resistance to lateral slipping and enhancing heat radiation while leaving continuous rib rows are left in the central portion of the tread as far as possible considering the wear resistance, therefore, it is attempted to arrange a pair of circumferential fine grooves for heat radiation in the central portion of the tread at a required minimum groove width.
In the off-the-road tire provided with the circumferential fine grooves for heat radiation, however, when the block rows are formed in the central portion of the tread as mentioned above, each block easily moves in front and behind or in left and right during the running of the tire under loading, so that the demand for high load capacity and high traction performance can not be satisfied and there are caused problems relating to the wear resistance such as premature wearing of tread, block chipping and other troubles.
On the other hand, when rows of land portion continuing more than necessary in the circumferential direction are formed on the central portion of the tread in such an off-the-road tire, if cut failure produced on the surface of the land portion row arrives at the belt layer, a large stress is produced resulting from the fact that the thick land portion rows are continuously arranged in the circumferential direction whenever the tread is largely deformed in the circumferential direction by subsequently large traction force, whereby there is caused a problem relating to the resistance to crack growth that the cut failure progresses as a crack between the tread and the belt in the circumferential direction of the tire along a propagation direction of such stress and further so-called cut separation of completely separating the tread from the belt is easily developed.
The invention is to solve the above problems inherent to the conventional technique and to provide an off-the-road tire capable of providing a high wear resistance without causing troubles such as premature wearing of the tread, block chipping and the like even under the exposure to severer load condition and traction condition, and capable of providing an excellent resistance to crack growth that even if cur failure is produced on a ground contact face of the tread, it hardly progresses into a crack between the tread and the belt layer.